The production of heavy oil and bitumen from a subsurface reservoir is quite challenging. One of the main reasons is that the viscosity of the oil is often greater than one million centipoise. Therefore, the removal of the oil from the subsurface is typically achieved by either surface mining or by the introduction of heat into the reservoir to lower the viscosity of the oil and thus allow it to be produced by the usual drilling and pumping techniques.
Thermal recovery has long been established as allowing the recovery of heavy oil and bitumen resources. It is well known the viscosity of oil is reduced when the oil is heated, and the viscosity of heavy oils can be reduced from millions of centipoise to 1-10 centipoise by injecting steam into the hydrocarbon reservoir. Cyclic steam stimulation (“CSS”) operations have been employed in heavy oil reservoirs around the world recovering millions of barrels of oil. Due to the extremely high viscosities of bitumen, cyclic steam operations have not been employed on a commercial scale due to the difficulty in initiating the recovery process and establishing commercial sustainable rates.
The steam assisted gravity drainage (“SAGD”) is an improved steam process that utilizes two horizontal wells vertically separated by approximately 5 meters. The process is initiated by circulating steam in both the wells to heat the heavy oil/bitumen between the well pairs via conduction until mobility is established, and gravity drainage can be initiated. Then the oil drains to the lower well, and is collected.
SAGD is one of the few commercial processes that will allow for the in-situ recovery of bitumen reserves. Due to the fact that the process requires steam and water treatment, a large capital investment in surface facilities is required, and a high operating expenditure or “OPEX” results. In addition, the product, heavy oil or bitumen, is sold at a significant discount to West Texas Intermediate (“WTI”, also known as Texas light sweet, a grade of crude oil used as a benchmark in oil pricing), providing a challenging economic environment when companies decide to invest in these operations. These conditions limit the resource that can be economically developed to reservoir thicknesses, typically thicker than 15-20 meters.
The primary driver for high costs is the steam to oil ratio, that is, the amount of steam that is required to produce 1 m3 or 1 barrel of heavy oil (bbl, 42 US gallons). During the recovery process, a well pair should be drilled and spaced such that it has access to sufficient resources to pay out the capital and operating costs. During the SAGD process, heat is transferred to the bitumen/heavy oil, as well as the overburden and underburden. In thinner reservoirs, economics do not allow wells to access sufficient resource, primarily due to high cumulative steam oil ratio “CSOR.” As a rule of thumb, a SOR of 3.0 is the typical economic limit used in the SAGD recovery today.
Solvent can improve SAGD operations by accelerating production and reducing SOR. When solvent is co-injected with steam in SAGD, different operators call the process by different names, but it most commonly known as Solvent-Aided Process (SAP), Expanded Solvent-SAGD (ES-SAGD), solvent assisted gravity drainage, etc. ES-SAGD improves the SAGD process by adding a second mechanism, the solvent dissolution into the bitumen, to the reduction of the viscosity of the bitumen. For example, the viscosity of the bitumen at 115° C. is 100 cp. By adding a little solvent to the system, the viscosity of the crude can be reduced to 5 cp.
Both SAGD and ES-SAGD are technologies that have shown success in the field. However, both exhibit opportunities for further optimization of operations and increase of economic value. One approach that may be used to do this is the incorporation of RF into these operations. This can be achieved by utilizing a subsurface antenna that is installed with existing wells or wells or one that is installed as a stand alone antenna.
Radio frequencies (RF) have been used in various industries for a number of years. One common use of this type of energy is the household cooking appliance known as the microwave (MW) oven.
Microwave radiation couples with, or is absorbed by, non-symmetrical molecules or those that possess a dipole moment, such as water. In cooking applications, the microwaves are absorbed by water present in food. Once this occurs, the water molecules rotate and generate heat. The remainder of the food is then heated through a conductive heating process.
Hydrocarbons do not typically couple well with microwave radiation. This is due to the fact that these molecules do not possess a dipole moment. However, heavy crude oils are known to possess asphaltenes, which are molecules with a range of chemical compositions. Asphaltenes are often characterized as polar, metal containing molecules. These traits make them exceptional candidates for coupling with radio frequencies. By targeting these molecules with RF radiation, localized heat will be generated which will induce a viscosity reduction in the heavy oil.
Through the conductive heating of the heavy crude oil or bitumen in place, a potential decrease in the startup time of a steam assisted gravity drainage (SAGD) operation or expanding solvent steam assisted gravity drainage (ES-SAGD) operation may be experienced. This may also lead to decreases in the amount of water required to produce the heavy oil, as well as a potential reduction green house gas emissions produced, both of which will have positive economic and environmental impacts on operations.
Additionally, the use of RF radiation in the presence of an alternate heat source can decrease the activation energy required for converting and breaking down carbon-carbon bonds. This synergistic effect can lead to the in situ upgrading of heavy crude oils by breaking down molecules that are known to significantly increase the viscosity of the crude oil. However, the use of RF frequencies in a reservoir is not straight forward, nor is the selection of the appropriate RF frequency easily accomplished.
U.S. Pat. No. 4,144,935 attempts to solve this problem by limiting the range in which radio frequencies are used to heat a particular volume in a formation. Such a method decreases the ability for one to use radio frequencies over a broad area and does not eliminate the problem of selecting the appropriate radio frequency to match the multitude of chemical components within the crude oil or bitumen. Furthermore, this method does not teach directing a radio frequency into a production well or bitumen formation to upgrade the heavy oil prior to the refinery process.
U.S. Pat. No. 5,055,180 attempts to solve the problem of heating mass amounts of hydrocarbons by generating radio frequencies at differing frequency ranges. However use of varying radio frequencies means that there are radio frequencies generated that are not efficiently utilized. In such a method one would inherently generate radio frequencies that have no effect on the heavy oil or bitumen. Furthermore, this method does not teach directing a radio frequency into a production well to upgrade the heavy oil before transporting to the refinery.
US20100294489 describe methods for heating heavy oil inside a production well. The method raises the subsurface temperature of heavy oil by utilizing an activator that has been injected below the surface. The activator is then excited with a generated microwave frequency such that the excited activator heats the heavy oil. However, the prior application uses higher frequency—0.3 gigahertz (GHz) to 100 GHz, and thus requires more energy to implement than the invention herein.
US20100294488 describes a method for preheating a formation prior to beginning steam assisted gravity drainage production. The method proceeds by forming a steam assisted gravity drainage production well pair within a formation. A preheating stage is then begun by injecting an activator into the formation. The preheating stage is then accomplished by exciting the activator with radio frequencies of 0.3 gigahertz (GHz) to 100 GHz. This is followed by beginning the steam assisted gravity drainage operation. However, the methods described herein also use the much higher frequency range, and thus are more energy intensive.
There thus still exists a need for an enhanced process that couples the use of non-microwave RF radiation to produce an upgraded hydrocarbon within a production well within a bitumen or heavy oil formation.